1. Field of Invention
This invention generally pertains to well logging, and more specifically to well logging apparatus and methods for rapidly developing data for determining such formation properties as resistivity. The apparatus and methods have general applications, but are particularly well suited for measuring while drilling applications employing advanced, high speed drilling apparatus.
2. Background of the Art
Resistivity is well known parameter used in evaluating earth formations surrounding a well borehole. In the oil and gas exploration and production industry, a measure of resistivity is used to delineate hydrocarbons from saline water within pore space of earth formation penetrated by the borehole. The basic principal underlying the measurement is that for a given formation matrix, the formation containing more resistive hydrocarbon fluid within the pore space will exhibit a greater composite resistivity than the same formation containing less resistive saline liquid within the pore space.
In the evolution of the art, resistivity instruments or xe2x80x9ctoolsxe2x80x9d were originally conveyed along the wellbore by means of a wireline cable. This technique is still widely used today. Resistivity related measurements are transmitted to the surface by means of the wireline for processing, interpretation and recording. This technique is applicable only in well boreholes that have been previously drilled.
In the petroleum industry, it is economically and operationally desirable to evaluate earth formations as they are being penetrated by a drill bit, rather than waiting until the entire well has been drilled as is required in conventional wireline logging. Apparatus and methods for evaluating formations while drilling became commercially available during the 1970s. This technology, known as measurement-while-drilling (MWD) or, alternately, logging-while-drilling (LWD), now includes a wide range of formation evaluation instrumentation which is typically mounted within a drill collar or a drill string, and conveyed along the borehole by the drill string during the drilling operation. Resistivity systems are included in the suite of available MWD systems. In addition to providing timely formation resistivity measurements while the well is being drilled, MWD resistivity measurements can be more accurate than their wireline counterparts. Well boreholes are typically drilled using drilling fluids at a pressure exceeding formation pressure. Over time, drilling fluid xe2x80x9cinvadesxe2x80x9d the formation in the vicinity of the borehole thereby perturbing composite resistivity measurements made with a tool within the borehole. Invasion is minimal at the time of drilling and typically increases over time after completion of the drilling operation. MWD resistivity measurements made during the actual drilling operation are, therefore, less perturbed by invasion than wireline resistivity measurements made after the well has been drilled. Invasion, and compensation for the effects of invasion, will be discussed in more detail hereafter.
Resistivity measurement tools typically include one or more transmitter coils and one or more receiver coils. Furthermore, more than one transmission frequency is typically used. Generally speaking, multiple transmitter and receiver coils, and multiple transmission frequencies are used to obtain composite resistivity measurements from differing radial depths into the formation in order to compensate for previously-mentioned drilling fluid invasion effects, to measure a wider range of resistivities, to resolve dipping formation beds, to measure formation anisotropy variables, and to measure distance to adjacent beds in geosteering drilling operations. Propagation type resistivity systems, which measure both phase shift and attenuation of transmitted signals, are widely used in prior art MWD systems. At present, this type of system is not used for wireline measurements, but their relative low cost, small physical size and high accuracy forms an attractive addition to the wireline logging arsenal of tools.
U.S. Pat. No. 5,581,024 to Meyer, Deady and Wisler discloses a depth correction and computation apparatus and methods for combining multiple borehole resistivity measurements. U.S. Pat. No. 5,594,343 to Clark, Wu, and Grijalva discloses resistivity well logging apparatus and methods with borehole compensation including multiple transmitters asymmetrically disposed about a pair of receiving antennas. U.S. Pat. No. 5,672,971 to Meador, Meisner, Hall, Thompson and Murphy discloses a resistivity well logging system arranged for stable, high sensitivity reception of propagating electromagnetic waves. U.S. Pat. No. 5,682,099 to Thompson, Wisler, and Schneider discloses a method for bandpass sampling in MWD systems, which is applicable to multiple frequency resistivity systems. This patent is intended to be incorporated herein by reference for disclosure such as the use of transmitters and receivers to garner information on the resistivity of the formation in the region of a wellbore. U.S. Pat. No. 5,892,361 to Meyer, Thompson, Wisler, and Wu discloses the use of raw amplitude and phase in propagation resistivity measurements to measure borehole environment. U.S. Pat. No. 5,329,235 to Zhou, Hilliker and Norwood discloses a method for processing signals from a MWD resistivity logging tool to enhance vertical resolution. There are other disclosures in the art, which discuss various configurations, frequencies, and processing methods of resistivity logging tools.
In prior art systems employing multiple transmission frequencies, measurements are made sequentially using one transmitter and one frequency at a time. Because of the relatively slow drilling penetration rates of earlier MWD measurement systems, the time consuming sequential multiple frequency transmission has not presented a significant vertical depth resolution problem. The industry is, however, evolving toward more and faster MWD measurements, especially when the measurements are made when the drill stem is being removed or xe2x80x9ctrippedxe2x80x9d from the borehole for purposes of changing a drill bit or for some other purpose. Sequential frequency transmission systems are detrimental to these faster methods. In addition, since wireline logging tools are conveyed along the borehole at a much faster rate than their MWD counterparts, sequential rather than simultaneous multiple frequency transmission is even more detrimental. No known prior art discloses a MWD resistivity logging system, which used multiple transmitter and receivers and multiple transmission frequencies that are transmitted simultaneously rather than sequentially.
In view of the prior art systems discussed above, an object of the present invention is to provide a propagation resistivity MWD logging system which employs at least two transmission frequencies transmitted simultaneously.
Another object of the present invention is to provide a MWD propagation resistivity logging system which utilizes at least two transmitters to transmit at least two different frequencies simultaneously.
Yet another object of the invention is to provide a MWD propagation resistivity logging system in which a single transmitter transmits at two different frequencies at the same time.
Still another object of the present invention is to provide a MWD propagation resistivity logging system employing at least two transmitters and two receivers which measure signals that are subsequently combined to yield phase difference and attenuation factor measurements that are compensated for adverse effect of systematic transmitter and receiver error.
Still another object of the invention is to provide a propagation resistivity measurement system that meets the above mentioned objects and that can be configured as a tool for wireline logging operations.
There are other objects and applications of the present invention that will become apparent in the following disclosure.
The present invention is a propagation resistivity system that utilizes one or more transmitter coil antennas, or xe2x80x9ctransmittersxe2x80x9d and at least two receiver coil antennas, or xe2x80x9creceiversxe2x80x9d. The system uses a wellbore resistivity tool or well-logging device which as illustrated may be embodied as a MWD tool, but can alternately be embodied as a wireline tool. The invention will be described using only two transmitters, two receivers, and two frequencies. Extension to three or more transmitters and/or frequencies is straight forward and would be understood by one of ordinary skill in the art. Two transmitters may be spaced equally on either side of two spaced-apart receivers. Each of the two transmitters transmits simultaneously. One transmitter operates on a high frequency, such as 2 megaHertz (MHz), which is an industry standard. The second transmitter operates at a lower frequency, which may be nominally as low as about 100 kiloHertz (kHz). The higher frequency signal penetrates a relatively shallow radial distance into the formation and the lower frequency penetrates to a radial depth which exceeds the penetration of the higher frequency. Composite measurements made at two radial depths are used to compensate for factors having adverse effects on resistivity measurements in the immediate region of the borehole. Such factors include invasion, variations in borehole size, variations in borehole fluid, and the like.
The higher frequency signal may be transmitted from the first transmitter T1, and the lower frequency signal may be transmitted simultaneously from the second transmitter T2 during a time interval ta. At a later time interval tb, the reverse occurs. That is, there is simultaneous transmission of the high frequency signal from transmitter T2 and the lower frequency signal from T1. Alternately, both high and low frequencies can be transmitted simultaneously from T1, and subsequently both high and low frequencies can be transmitted simultaneously from T2. In either embodiment, two frequencies are transmitted simultaneously from the tool to propagate into the formation and to produce signals, which are subsequently detected by the receivers.
Using the first transmission sequence, the high frequency or first frequency signal from T1 is received at the closer spaced receiver R1 with a phase xcfx86111 measured in degrees or radians and relative to the phase of the transmitted signal (the first number after the xcfx86 indicating the transmitter from which the signal originated, the second number after the xcfx86 indicating the signal is received by the first receiver, and the third number indicating the signal is propagated at a first frequency). The high or first frequency signal from the first transmitter T1 is also received at the first receiver R1 having been attenuated relative to the transmitter signal by an amount xcex1111 measured in decibels or nepers. The numbers after the xcex1 indicate the same as the three numbers after the phase. Simultaneously, phase and attenuation of the signal from transmitter T1 is received at receiver R2, at frequency 1, xcfx86121 and xcex1121. It is well known in the art that the phase difference xcfx86121-xcfx86111 and the attenuation difference xcex1121-xcex1111 are functions of formation properties and conditions in the vicinity of the borehole and receiver antennas, and may be defined as a phase difference xcex94xcfx8611 and attenuation difference xcex94xcex111. In each case the first number after the xcex94xcfx86 or xcex94xcex1 indicates from transmitter T1 and the second number indicates at frequency 1. In particular the phase difference and attenuation difference are functions of resistivity of the formation. T2 simultaneously transmits a signal at the lower frequency, denoted by the subscript 2, which is received by R2 and by R1 thereby defining a phase difference xcex94xcfx8622 and attenuation difference xcex94xcex122. Next in the measurement sequence T2 then transmits the high frequency, which is received at R2 and R1 and thereby defines a phase and attenuation difference xcex94xcfx8621 and xcex94xcex121. T1 simultaneously transmits a signal at the lower frequency which is received by R1 and R2, thereby defining a phase and attenuation difference xcex94xcfx8612 and xcex94xcex112. The terms xcex94xcfx8611 and xcex94xcfx8621 are combined to yield a compensated phase difference xcex94xcfx86C1=(xcex94xcfx8611+xcex94xcfx8621)*xc2xd. Similarly the terms xcex94xcex111 and xcex94xcex121 are combined to yield a compensated attenuation difference xcex94xcex1C1=(xcex94xcex111+xcex94xcex121)*xc2xd. And in like manner the terms xcex94xcfx8612, xcex94xcfx8622, xcex94xcex112, and xcex94xcex122 are combined to yield compensated phase difference xcex94xcfx86C2=(xcex94xcfx8612+xcex94xcfx8622)*xc2xd and attenuation difference xcex94xcex1C2=(xcex94xcex112+xcex94xcex122)*xc2xd.
Transmission is switched from transmitter T1 to T2 and back again to transmitter T1 in the first embodiment of the invention. In the second embodiment T1 simultaneously emits high and low frequencies, and next in the measurement sequence T2 simultaneously emits high and low frequencies. Compensated values xc3x8CH, xc3x8CL, as well as xcex1CH and xcex1CL, are computed in the same manner.
As is well known in the industry compensated values xcex94xcfx86C1, xcex94xcfx86C2, xcex94xcex1C1, and xcex94xcex1C2, are then used separately and/or combined to determine formation resistivities, and subsequently formation hydrocarbon saturation, despite the effects of invasion, borehole fluids, and systematic equipment error.